The invention relates to a display device comprising a first substrate provided with row electrodes and a second substrate provided with column electrodes, in which overlapping parts of row and column electrodes with an interpositioned layer of electro-optical material define pixels, said electro-optical layer being capable of assuming a plurality of states, at least two states of which are stable in the absence of an electric field, further comprising drive means for driving the row electrodes with selection signals and for driving the column electrodes with data signals in conformity with an image to be displayed.
Particularly, the electro-optical layer comprises a chiral-nematic liquid crystal material which can assume a plurality of states, at least one focal-conic state and a planar state of which are stable in the absence of an electric field.
More generally, the invention relates to a display device in which an electro-optical layer is switchable between a plurality of (long lasting) stable states. A display device based on two (or more) stable states may be used in various applications, for example, if information which is written once is to be maintained for a longer period of time (electronic newspapers, telephony, smart cards, electronic price tags, personal digital assistants, billboards, etc.).
A display device of the type mentioned above is described in WO 98/50804.
A display element (pixel) in such a display device, based on a chiral-nematic liquid crystal material, has several stable states, namely a transmissive state, which corresponds to the focal-conic state of a layer of liquid crystal material, and a reflective state, which corresponds to the planar state of the layer of liquid crystal material. The color (wavelength) of the reflected light is dependent on the pitch of the liquid crystal material, i.e. the distance across which the director (the average orientation of the molecules in a layer) makes a twist of 360 degrees. In the absence of an electric field, both states are stable for a long period of time. In the transmissive state, light of said color is transmitted to a greater or lesser degree, dependent on the texture (ratio between parts of a pixel in the planar and the focal-conic state, respectively). Moreover, such a display device may also be in a homeotropic state; at a high voltage, all molecules (directors) are directed towards the field. Incident light then passes the liquid crystal material unhindered. When used without polarizers, the color in the homeotropic state is determined by the color of the background, for example, an absorbing layer in a reflective display device. The display device is usually only brought to this state in order to reach one of the two stable states. Dependent on the frequency used and on the voltage of the switching pulses, a pixel changes to either the focal-conic or the planar state.
The selection time (addressing time) for writing the different states is usually rather long. Without special measures, it is 20 to 30 msec, which is too long for use in, for example, an electronic newspaper.
Said patent specification describes how the addressing time which is necessary to reach the different states can be reduced by means of a special drive mode and by means of a preparation phase and an evolution phase.
A problem in these types of display devices is the realization of intermediary transmission (reflection) levels or grey values. These are usually not stable.
It is an object of the invention to obviate this drawback completely or partly. More particularly, it is an object of the invention to provide a display device of the type described, in which stable grey values are realized. To achieve this, a display device according to the invention is characterized in that, in the operating state, the drive means provide a row electrode during a selection period with at least one pulsatory selection signal comprising a first sub-selection signal having a first amplitude and a second, opposite, sub-selection signal having substantially the same amplitude and duration, and, when the row electrode is being supplied with the first sub-selection signal, the drive means supply the column electrodes with a sub-column signal having a second amplitude and, when the row electrode is being supplied with the second sub-selection signal, the drive means supply the column electrodes with an opposite sub-column signal having substantially the same second amplitude and duration, the duration of the sub-column signals defining the grey value.
The word xe2x80x9coppositexe2x80x9d is herein understood to mean that the amplitudes of the (sub-)signals are substantially identical but opposite to a certain reference level, for example, ground.
A preferred embodiment is characterized in that, in the operating state, the drive means provide a row electrode during a selection period with at least one pulsatory selection signal having a first amplitude and a first pulse width, and supply the column electrodes with data signals having substantially the same first pulse width and a second amplitude, the phase difference between the selection signal and the data signal defining a grey value at the location of a pixel. It is true that WO 98/50804 states the possibility of realizing grey values, but for this purpose use is made of time modulation or amplitude modulation. However, it appears that the defining RMS voltage varies during non-selection, dependent on voltages supplied to other elements (here, notably the voltage during the evolution phase is concerned). This variation of the RMS voltage during non-selection affects said texture (ratio between parts of a pixel in the planar and the focal-conic state, respectively), so that the grey values are also dependent on data signals supplied during selection of pixels in other rows. By making use of phase modulation according to the invention, this influence is prevented.